U.S. patent application number 11/814473 was filed with the patent office on 2010-02-04 for security holograms.
Invention is credited to John David Wiltshire.
Application Number | 20100027082 11/814473 |
Document ID | / |
Family ID | 36586016 |
Filed Date | 2010-02-04 |
United States Patent
Application |
20100027082 |
Kind Code |
A1 |
Wiltshire; John David |
February 4, 2010 |
SECURITY HOLOGRAMS
Abstract
This invention relates to holograms employing colour addition
techniques, and to methods and apparatus for the fabrication of
such holograms. The holograms are particularly useful for security
applications. A volume reflection hologram storing at least two
images, the hologram comprising: a first stored image configured to
replay at a first wavelength; a second stored image configured to
replay at a second wavelength different to said first wavelength;
wherein said first and second images at least partially overlap
when replayed together; and wherein said first and second
wavelengths are selected such that where said first and second
images overlap they give the appearance of a colour defined by a
third wavelength different to both said first and second
wavelengths.
Inventors: |
Wiltshire; John David;
(Cambridgeshire, GB) |
Correspondence
Address: |
CHRISTOPHER P. HARRIS;TAROLLI, SUNDHEIM, COVELL & TUMMINO, LLP
1300 EAST NINTH STREET, SUITE 1700
CLEVELAND
OH
44114
US
|
Family ID: |
36586016 |
Appl. No.: |
11/814473 |
Filed: |
January 23, 2006 |
PCT Filed: |
January 23, 2006 |
PCT NO: |
PCT/GB2006/050017 |
371 Date: |
March 30, 2009 |
Current U.S.
Class: |
359/2 ;
359/24 |
Current CPC
Class: |
G03H 1/2249 20130101;
G03H 1/24 20130101; G03H 2001/2231 20130101; G03H 2001/186
20130101; G03H 2001/0016 20130101; G03H 2210/22 20130101; B42D
25/328 20141001; G03H 2210/20 20130101; G03H 1/0248 20130101; G03H
2222/13 20130101; G03H 2001/2263 20130101; B42D 25/29 20141001;
G07D 7/0032 20170501; G03H 2001/2213 20130101 |
Class at
Publication: |
359/2 ;
359/24 |
International
Class: |
G03H 1/28 20060101
G03H001/28; G03H 1/00 20060101 G03H001/00 |
Foreign Application Data
Date |
Code |
Application Number |
Jan 21, 2005 |
GB |
0501212.5 |
Feb 11, 2005 |
US |
60/651651 |
Claims
1-11. (canceled)
12. A volume reflection hologram storing at least two images, the
hologram comprising: a first stored image configured to replay at a
first wavelength; a second stored image configured to replay at a
second wavelength different to said first wavelength; wherein said
first and second images at least partially overlap when replayed
together; and wherein said first and second wavelengths are
selected such that where said first and second images overlap they
give the appearance of a colour defined by a third wavelength
different to both said first and second wavelengths.
13. A volume reflection hologram as claimed in claim 12 wherein
said colour of said third wavelength is substantially visually
indistinguishable from a colour defined by a mixture of said first
and second wavelengths.
14. A volume reflection hologram as claimed in claim 12 wherein
said first wavelength defines a green colour, said second
wavelength defines a red colour, and said third wavelength defines
a yellow colour.
15. A volume reflection hologram as claimed in claim 14 wherein
said third wavelength is between 570 nm and 580 nm.
16. A volume reflection hologram as claimed in claim 12 wherein
said first wavelength defines a green colour; said second
wavelength defines a blue colour, and said third wavelength defines
a cyan colour.
17. A volume reflection hologram as claimed in claim 16 wherein
said third wavelength is between 490 nm and 510 nm.
17. A volume reflection hologram as claimed in claim 16 wherein
said third wavelength is between 490 nm and 510 nm.
18. A volume reflection hologram as claimed in claim 12 wherein
said first and second images substantially correspond to give a
visual appearance to a human of a single, combined image at said
colour of said third wavelength.
19. A security document or bank note incorporating the volume
reflection hologram of claim 12.
20. A method of fabricating a security hologram, the method
comprising: storing a first image in said hologram, said first
image being configured to replay at a first wavelength; and storing
a second image in said hologram, said second image being configured
to replay at a second wavelength, when replayed said second image
at least partially overlapping said first image; wherein said first
wavelength has a first colour and said second wavelength has a
second colour; and wherein a mixture of light at said first and
second wavelengths gives the appearance to the human eye of a
spectral colour corresponding to light of a third wavelength
different to said first and second wavelengths, whereby said
overlapping portions of said first and second images give the
appearance of a stored image portion configured to replay at said
third wavelength.
21. A security hologram storing a first image configured to replay
at a first wavelength, and storing a second image configured to
replay at a second wavelength, when replayed said second image at
least partially spatial overlapping said first image; wherein said
first wavelength has a first colour and said second wavelength has
a second colour; and wherein a mixture of fight at said first and
second wavelengths gives the appearance to the human eye of a
spectral colour corresponding to light of a third wavelength
different to said first and second wavelengths; whereby said
overlapping portions of said first and second images give the
appearance when replayed of a stored image replayed at said third
wavelength.
22. Apparatus for fabricating a volume reflection hologram, the
apparatus comprising: means for writing a first image into said
hologram to replay at a first wavelength; means for writing a
second image in said hologram to replay at a second wavelength,
said second image and said first image to an observer at least
partially spatially overlapping; and wherein said first and second
wavelengths are defined such that a mixture of light at said first
and second wavelengths gives the appearance to the human eye of a
spectral colour corresponding to fight of a third wavelength
different to said first and second wavelengths.
Description
[0001] This invention relates to holograms employing colour
addition techniques, and to methods and apparatus for the
fabrication of such holograms. The holograms are particularly
useful for security applications.
[0002] Holograms have many uses but one increasingly important
application is that of security, where a hologram may be used as an
anti-counterfeiting device on security documents such as passports,
visas, identity cards, driver licenses, government bonds, Bills of
Exchange, banknotes and the like, as well as on packaging and
labelling. To improve security special visual effects may sometimes
be employed such as kinetic effects, for example the
appearance/disappearance of graphic elements (sometimes termed
Kinegram.TM.), or contrast/brightness variation effects, for
example a graphic converting from a positive to a negative image (a
Pixelgram.TM.).
[0003] It will be appreciated, however, that there is scope for
improved holographic techniques which contribute to increased
security or which exhibit other desirable traits such as increased
brightness and/or an improved visually aesthetic appearance.
Background information relating to improved techniques for
multicolour reflection holograms can be found in Improved
Techniques for Multicolour Reflection Holograms, P. Hariharan, 1980
J. Opi, 11, 53-55, which refers to advantages of recording
different colours in separate plates so that when the plates are
aligned a multicolour image is the result.
[0004] In this specification we are particularly concerned with
volume reflection holography. Broadly speaking a reflection
hologram is a hologram which is constructed by interfering object
and reference beams which are directed onto a recording medium from
opposite sides of the medium; a volume hologram is a hologram in
which the angle difference between the object and reference beams
is equal to or greater than 90 degrees. Volume holograms are
sometimes referred to as "thick" holograms since, roughly speaking,
the fringes are in planes approximately parallel to the surface of
the hologram, although in practice the thickness of the recording
medium can vary significantly, say between 1 .mu.m and 1.00 .mu.m,
typically around 7 .mu.m.
[0005] Volume holograms are particularly useful for security
applications as they are difficult to copy, although they can also
be difficult to mass produce. One property of volume holograms is
that when illuminated with white light at the correct angle they
replay one or more stored images in a particular wavelength,
determined by the spacing of a corresponding set of fringes stored
within the hologram recording material. If the viewing angle is
changed the brightness of the replayed image or images changes as
the diffraction efficiency changes but the colour remains
substantially the same. At wavelengths where the eye is
particularly colour sensitive a small colour shift can sometimes be
observed on tilting the hologram but it is nonetheless possible to
define a colour for the replayed image (and corresponding
wavelength) as being that associated with the centre or peak
viewing angle at which the image is at its brightest; in practice
the colour of a replayed image is clear and easy to see.
[0006] It is possible to store a plurality of different images
within a single volume hologram and each of these images may be
recorded such that it replays at a different wavelength; in this
way it is possible to store, say, a red and green image within a
single hologram. This can either be done by recording using lasers:
of corresponding colours, in the foregoing example red and green
lasers, or by chemical and/or physical shifting of the wavelength
of a recorded image. For example, a gel-based recording material
may be pre-swollen in a humidity cabinet, exposed, then dried to
shrink the hologram before another image is recorded (or
vice-versa); or alternatively chemical processing may be employed
to add or remove material from a written hologram to change the
fringe spacing, for example increasing spacing by trapping material
within the recording medium by polymer cross-linking. Such
techniques are well known to the skilled person.
[0007] The applicants have observed experimentally that some
combinations of colours, when replayed by a hologram, can give
substantially the same visual appearance as that of a single,
spectrally-pure colour, and have recognised that this may be used
to increase the security of a recorded hologram. More particularly
the applicants have recognised that, because of the characteristics
of human vision, where say red and green reflection holograms
overlap the eye perceives the result as a unified yellow hologram
rather than separate reddish green or greenish red images. Such a
unified image appears to an observer to have been created using
pure (single wavelength) yellow light and it has been recognised
that this effect can be used to provide, for example, increased
security against counterfeiting.
[0008] According to a first aspect of the present invention there
is therefore provided a volume reflection hologram storing at least
two images, the hologram comprising: a first stored image
configured to replay at a first wavelength; a second stored image
configured to replay at a second wavelength different to said first
wavelength; wherein said first and second images at least partially
overlap when replayed together; and wherein said first and second
wavelengths are selected such that where said first and second
images overlap they give the appearance of a colour defined by a
third wavelength different to both said first and second
wavelengths.
[0009] Broadly speaking, in embodiments when the first and second
stored images are replayed they give rise to light of a mixture of
the first and second wavelengths and this colour or wavelength
mixing gives the appearance of a colour of a substantially
spectrally pure third wavelength where the first and second images
overlap. In preferred embodiments the colour of the third
wavelength is substantially visually indistinguishable from a
colour defined by a mixture of light at the first and second
wavelength. Fabricating a hologram in this manner contributes to
increased security since, to a casual observer, it appears as
though (at least in the overlapping image portions) the hologram
has been recorded using light of a single spectrally pure colour at
this third wavelength whereas in fact two images have recorded at
two different wavelengths. This is, however, distinguishable to a
machine hologram reader which can, for example, look for the
presence of the two colours separately.
[0010] Preferably the first and second images are both
substantially planar, and preferably both occupy substantially the
same plane when replayed. Preferably there is substantially no
angular separation between the first and second images, although
the appearance of the images together may change with viewing
angle, for example changing in brightness or (a little) in
perceived colour. In preferred embodiments the first and second
images substantially correspond, to give a visual appearance (to a
human observer) of a single, combined image at the colour of the
third wavelength.
[0011] In one embodiment the first and second wavelengths define
green and red colours respectively and the third wavelength defines
a yellow colour; preferably this yellow colour is between 570 nm
and 580 nm. For example a colour very similar to the yellow
observed at 575 nm may be achieved by a combination of a green
laser at 550 nm (for example a frequency-doubled WAG laser) and a
red laser at 647 nm (Krypton red) or 633 nm (Helium neon). In
another example the first and second wavelengths respectively
define green and blue colours and the third wavelength define a
cyan colour, in particular of a wavelength of 490 nm and 510 nm.
Thus, for example, a combination of a 514 nm (Argon green) laser
and a 488 nm (Argon blue) laser used to record corresponding images
into a volume reflection hologram can result in a combined,
replayed image which has a colour visually very similar to that of
spectral cyan at 500 nm (in other arrangements Argon 455 nm may be
employed in place of Argon 488 nm). The skilled person will readily
understand that many other combinations of wavelengths may be
employed, using routine experimentation to determine which
combinations result in holographic images which where coincident
and replayed at the same time give the appearance of having been
recorded at a single spectral colour different from either of the
actual wavelengths of the stored images. It will further be
appreciated that this general principle may be extended to three or
more stored images each recorded such that it replays the at a
different wavelength, the three (or more) combined wavelengths
being selected to give the appearance, when mixed, of a
substantially spectrally pure colour.
[0012] The mechanism of the colour mixing process within the human
eye is speculatively considered to result from the way in which the
cells in the human retina are wired up. For example, the I-cones,
which respond to greenish hues, and the L-cones which respond to
reddish hues, are wired in opposition so that an L-cone inhibits an
I-cone.
[0013] Embodiments of the above described hologram may be
fabricated in a conventional manner using, for example, lasers of
different wavelengths. Potentially, however, the recording process
could be arranged so that the first and second images combine,
effectively, in different proportions, for example replaying at
different brightnesses, optionally providing an additional
parameter which may be adjusted, say, to tune to a desired third
wavelength colour match.
[0014] The invention further provides a method of fabricating a
security hologram, the method comprising: storing a first image in
said hologram, said first image being configured to replay at a
first wavelength; and storing a second image in said hologram, said
second image being configured to replay at a second wavelength,
when replayed said second image at least partially overlapping said
first image; wherein said first wavelength has a first colour and
said second wavelength has a second colour; and wherein a mixture
of light at said first and second wavelengths gives the appearance
to the human eye of a spectral colour corresponding to light of a
third wavelength different to said first and second wavelengths;
whereby said overlapping portions of said first and second images
give the appearance of a stored image portion configured to replay
at said third wavelength.
[0015] The invention also provides a security hologram storing a
first image configured to replay at a first wavelength, and storing
a second image configured to replay at a second wavelength, when
replayed said second image at least partially spatial overlapping
said first image; wherein said first wavelength has a first colour
and said second wavelength has a second colour; and wherein a
mixture of light at said first and second wavelengths gives the
appearance to the human eye of a spectral colour corresponding to
light of a third wavelength different to said first and second
wavelengths; whereby said overlapping portions of said first and
second images give the appearance when replayed of a stored image
replayed at said third wavelength.
[0016] Preferably the security hologram comprises a volume
reflection hologram. Optionally one or both of the stored images
may comprise a biometric image such as an image of an iris or
fingerprint.
[0017] The invention further provides apparatus for fabricating a
volume reflection hologram, the apparatus comprising: means for
writing a first image into said hologram to replay at a first
wavelength; means for writing a second image in said hologram to
replay at a second wavelength, said second image and said first
image to an observer at least partially spatially overlapping; and
wherein said first and second wavelengths are defined such that a
mixture of light at said first and second wavelengths gives the
appearance to the human eye of a spectral colour corresponding to
light of a third wavelength different to said first and second
wavelengths.
[0018] The means for writing the first and second images may
comprise, for example, a spatial light modulator such as an LCD
screen coupled to an image storage/display system to display images
to be written into the hologram on the screen, the screen
modulating one of two interfering light beams used to create the
hologram. Related techniques are described in more detail in the
applicant's co-pending Application No. PCT/GB2004/050014 filed 1
Oct. 2004, priority date 1 Oct. 2003, the contents of which are
hereby incorporated by reference.
[0019] These and other aspects of the invention will now be further
described, by way of example, with reference to the accompanying
figures in which:
[0020] FIG. 1 shows a security document incorporating a hologram
according to an embodiment of one aspect of the present invention;
and a flow diagram for the fabrication of the data carrier of FIG.
1a;
[0021] FIGS. 2a and 2b show, respectively, a flow diagram of a
hologram fabrication method according to an embodiment of another
aspect of the present invention, and apparatus for implementing the
method;
[0022] FIG. 3 shows a computer control system for the apparatus of
FIG. 2b;
[0023] FIGS. 4a to 4c show details of a holographic writer and
first and second alternative holographic film supports;
[0024] FIG. 5 shows a schematic diagram of an optical arrangement
for the apparatus of FIG. 2b;
[0025] FIG. 6 shows the sensitivity of the human eye in terms of
its three sets of cones;
[0026] FIGS. 7a to 7c show covert text message incorporation in the
surface of a security hologram;
[0027] FIGS. 8a to 8c show covert security pattern incorporation in
the surface of a security hologram;
[0028] FIGS. 9a to 9c show, respectively, white, red/green, and
yellow replayed images of a volume reflection hologram of FIG.
7;
[0029] FIG. 10 shows a schematic view of a vertical cross-section
through an embodiment of a hologram according to the invention
showing sets of "red" and "green" fringes, according to an
embodiment of the present invention:
[0030] FIG. 11 shows red and green replayed images of an embodiment
of a hologram according to the invention together with their
reflection spectrum; and
[0031] FIG. 12 shows yellow replayed images of an embodiment of a
hologram according to the invention together with their reflection
spectrum.
[0032] Referring to FIG. 1, a security document 10 comprises a
hologram 14 storing biometric and other data and text 16 such as a
name, address, national security number and the like. Optionally,
depending upon the type of document (for example with, say a credit
card) an integrated circuit memory chip 12 may also be included, as
described in more detail in the Applicant's PCT/GB2004/050014
(ibid). Document 10 may comprise, for example, an identity card or
document, driving license, passport, credit card or any other form
of identification.
[0033] Referring to FIG. 2a a hologram for card 10 may be created
by capturing biometric information such as a fingerprint (step 20),
taking this a first image (step 22), and copying this to create a
second image (step 24), which at least partially overlaps the first
and may be substantially the same as the first. Optionally other
data may be created or input for storage with the hologram. At step
26 the first and second images are written into a reflective or
reflection hologram, which is then processed in a conventional
manner to fix the images (step 28), together with any additional
data stored. The hologram is then attached to an identity document
and covered with a protective layer (step 32).
[0034] FIG. 2b shows a holographic recording system. Data for
recording with the hologram may be entered into the terminal and,
optionally records such as write once read many (WORM) records are
created locally and also, via a network, at a remote database for
auditing and verification purposes. The film is preferably held
securely within the hologram writer, for example accessed by a
mechanical key, so that a secure film box can be removed from the
writer and sent for secure chemical processing and incorporation
into a document.
[0035] Referring next to FIG. 3, this shows a block diagram of a
computer control system for the apparatus of FIG. 2a. Biometric
data such as a fingerprint image is captured by commercial off the
shelf equipment such as the BAC Securetouch USB2000 available from
Bannerbridge plc of Basildon, UK and provided to an image processor
302 which, under control of a control processor 304, provides an
image to display driver 306 for display on an LCD display 308, for
example at SVGA resolution, at a size of approximately 30 mm.sup.2.
The captured input image may be converted into a binarised image,
and positive and negative versions of the image may be generated,
either by image processor 302 or by control computer 304.
[0036] The size and resolution of the display may be determined
based upon processing power and cost. The LCD display acts as a
spatial light modulator as described below with reference to FIG.
4a and thus preferably allows illumination through the device.
Typically such a display comprises a micrometer thick sheet of
polarising material followed by electrically configurable liquid
crystal material. The LCD display may be of a type which has
permanently on or off pixels rather than pixels which are
refreshed, for example a ferroelectric liquid crystal device so
that the pixels stay in either an on or an off (black or white)
state for the duration of the image recordal, typically around two
seconds. Alternatively a conventional, raster scanned display may
be employed. A suitable LCD display is available from Central
Research Laboratories Ltd of London, UK, for example model SVGA2
monochrome transmission LCD. An LCD display without an in-built
polariser may be employed with plane polarised laser illumination,
which in effect provides approximately 50% more light.
[0037] The two images for simultaneous replay by the volume
reflection hologram are preferably recorded using different
coloured lasers. Other means of creating the two different colours
in the hologram include chemical or physical expansion of the film
layer prior to exposure, and adjustment of the final thickness of
the hologram layer during chemical processing of the film. For
example the developer and bleaching solution for silver halide
materials may be designed/selected to produce the desired colours
in the final image. The layer properties of the selected recording
film also affect the colour reconstruction of the stored
images.
[0038] The hologram recording medium may comprise any conventional
hologram recording medium including, but not limited to,
dichromated gelatine (DCG), silver halide, and photo-polymer based
materials.
[0039] Referring next to FIG. 4a this shows an optical
configuration of the spatial light modulator and film. The spatial
light modulator may be substantially adjacent the film or may be
spaced apart from the film by a glass or quartz spacer. Spacers of
2, 4 or 6 mm may be employed, optionally mechanically selectable oh
the control of the computer controller 304 in order to record
images at different planes within the hologram. The maximum
adjustment of the spacing between the spatial light modulator and
film is determined by the coherence length of the laser, and is
typically a few mm to a few cm (say in the range 1 mm to 30 mm,
possibly up to 100 mm) for a diode laser (since, as shown later in
FIG. 5, optical path lengths from the laser for the object and
reference beams are preferably substantially matched).
[0040] Preferably the arrangement includes a diffuser prior to the
spatial light modulator comprising, for example, ground glass or
substantially non-birefringent plastic material such as
polycarbonate or polyester film. Such diffusers are available from
Lee Filters in the UK. The diffuser does not destroy the hologram
since the differences in optical path lengths to the film from
diffused rays originating from a point on the diffuser is very
small, but the diffuser has the effect of providing a hologram with
a speckle pattern rather than a so-called shadowgram which appears
shiny like a mirror.
[0041] Many mechanical schemes may be employed for holding the film
in close proximity to the spatial light modulator or spacer
depending, for example, on whether sheet fed or roll fed film is
employed. FIGS. 4b and 4c show two examples of film transport
mechanisms; for sheet film a sheet feeder may be employed;
optionally a vacuum chuck may also be used to ensure the
holographic recording material bears against the spatial light
modulator or spacer. In a less preferred arrangement a mounting
frame holds the SLM and/or spacer in a fixed or controllable
spatial relationship with respect to the film. In any of the above
arrangements index matching or interface coupling temporary
adhesive may be employed if necessary.
[0042] FIG. 5 shows, one example of an optical configuration for
the apparatus of FIG. 2b. In particular this optical configuration
shows how either laser A (say; red, for example Krypton 647 nm or
HeNe 633 nm) or laser B (say, green, for example a 532 nm frequency
doubled Nd-YAG laser, or 550 nm dye laser) may be selected for
recording the respective first and second stored images in a volume
reflection hologram, by tilting a mirror between two alternative
positions, for example under servo control. In this way two
different images can be recorded, each with a different colour (red
and green), but both in substantially the same plane (when
replayed) with reference to the plane of the recording material.
Where the two images are coincident, on replay, rather than
separate red and green images being seen the visual appearance is
that of a distinct colour, yellow, having neither a reddish nor a
greenish tint. This yellow approximately corresponds to a
spectrally pure 575 nm yellow.
[0043] When defining whether two colours are visually
distinguishable (to a human observer) the CIE (Commission de
L'Eclarage) difference between two colours, .DELTA.E, may be
employed. Here:
.DELTA.E=
((.DELTA.L*).sup.2+(.DELTA.a*).sup.2+(.DELTA.b*).sup.2)
where the L*, a* and b* values represent the lightness,
red-to-green and blue-to-yellow values of the two colours. Thus a
.DELTA.E value of 1 or more may be taken as visible (.DELTA.E equal
to or less than 1.0 assumed not visible), or a .DELTA.E of 0.1 may
be taken as not corresponding to a visible colour difference, and
.DELTA.E equal to or greater than 0.1 assumed not visible (although
.DELTA.E does not exactly correspond to a visual assessment of a
colour difference for all colours). In embodiments two colours are
considered visually distinguishable if they have a CIE colour
difference of <0.1, <0.5, or <1.0.
[0044] In FIG. 6, the sensitivity of the human eye is shown in
terms of its three sets of cones, which it is believed are
responsible for the perception of colour. Unlike the highly
monochromatic behaviour characteristic of thick volume reflection
holograms, these colour detectors in the eye each have a broad
range of sensitivity.
[0045] Accordingly, an orange or yellow stimulus will affect both
the `green` and `red` cones, so that the brain is basically unable
to distinguish between illumination by simultaneous red and green
irradiation, and a single source in the wavelength range central to
those colours, for example in the area of 575-595 nm.
[0046] It is clear from the curve for the green cones that these
have a higher total sensitivity than the red, and especially the
blue cones. However, the hologram origination system can cope with
satisfying this sensitivity imbalance of the eye, by allowing for
higher efficiency of the recording of the red grating component,
than for the green grating component.
[0047] A similar effect can by achieved at the blue end of the
visible spectrum. A chosen `cyan` equivalent single spectral colour
between say 495-505 nm can be selected such that its visual
stimulus affects both the blue and green cones in a way
indistinguishable to the brain from the effect of a pair of stimuli
acting at say 458 and 532 nm. The adjustment of all of these
colours en bloc to the most effective centre of wavelength to
optimally facilitate the required effects can be achieved by
judicious selection of laser and chemistry techniques.
[0048] In one embodiment of the technique, diffractive lettering
perceived by the human eye as yellow in colour, achieved by the
mixing of red and green light, occupies space within a land area
perceived as a similar colour, but in this case achieved by a
narrow-band reflective diffraction of a pure spectral yellow colour
of a wavelength between 575 and 595 nm for example.
[0049] The exposure of the holographic image components may
achieved with the use of various lasers, such as HeNe 633 nm and
Nd:YAG second harmonic 532 nm and a Krypton 568 nm or HeNe 594 nm,
or alternatively be chemical or physical pre- or post-treatment of
the recording layer as has been known in the art, or by a mixture
or combination of these methods.
[0050] Alternatively, colours at the opposite end of the visible
spectrum could be used for the purpose. Thus Argon lasers at 458 nm
or 488 nm could be used to produce blue components to achieve
mixing from Argon 514 nm or Nd:YAG second harmonic at 532 nm. The
result is that blue and green colour components exposed by the blue
argon lines and the Nd:YAG are mixed in the body of the lettering
and have a very similar colour to land areas exposed at 514 nm and
chemically moved to replay at 500-510 nm which corresponds to a
spectral equivalent of the cyan mixture.
[0051] Advantageously, this text may be very small, e.g. less than
1 mm high, and thus may be even less apparent to the casual
observer viewing the hologram in white light illumination.
[0052] Landis and Gyr have a "microtext" technique in their
"kinegram" diffraction grating security systems to provide a secret
security advantage by the use of lettering so small that a small
lens is required to verity the text, which is covert solely be its
minuscule dimensions.
However, the revelation of the covert code, by the use of
illumination of a single spectral colour, which by virtue of the
narrow bandwidth of the reflective diffraction associated with
thick volume holograms, provides now a powerful interrogation
system for the security hologram at very low cost.
[0053] Such illumination could, but need not necessarily, be
provided by laser since no special coherence qualities are required
to reconstruct the Lippmann hologram, so that monochromatic LED, or
filtered white light can be used to illuminate the finished
security hologram and reveal the covert information.
[0054] In FIG. 7, the diagram 7a shows a schematic of the intended
covert text message incorporation in the surface of a security
hologram. The FIGS. 7b and 7c show the artwork separations used for
separate image exposures in the two visually substantially
indistinguishable colours.
[0055] FIG. 8a shows a similar schematic in the case where the
covert security code is in the form of a pattern rather than a text
message. Such a surface pattern could typically form the surface
surround to a main overt image component such as a corporate logo,
which may typically be recorded in a completely separate
contrasting colour, with a view to being an eye catching and
visually dominant feature of the holographic security device.
[0056] It is common within the hologram security industry, to
provide within a single hologram a number of separate component
images to provide a range of visual and covert features which can
be used for verification by the layperson, a trained observer, or
at the forensic level in the laboratory, to establish whether a
security device is genuine. This method is ideally suited to this
style of design.
[0057] In FIG. 9a, the effect of illuminating the hologram with
artwork derived from FIG. 7, with white light is shown when viewed
by the human eye without any filter devices. The whole hologram
surface appears to illuminate in a single homogeneous yellow or
cyan colour and the observer detects little or no detail within the
surface.
[0058] However, the effect of illuminating with either red (630-670
nm) or green (530-560 nm) or pure spectral yellow 575-595 nm is
that the text is revealed with a positive illumination as in FIG.
9b or the land or background area is illuminated as in FIG. 9c to
reveal the covert, text information.
[0059] Alternatively, the use of white light incident light is
possible such that the code is revealed only when viewed through
narrow bandpass filters in red, green or yellow such that component
of the selectively diffracted light are attenuated or
transmitted.
[0060] In FIG. 10, the difference is demonstrated between the
holographic fringe microstructures in the zones of the holographic
image, relating to the single wavelength and twin wavelength
recording zones of the hologram. The fringes shown equate to
modulation of the refractive index and it will be imagined that the
twin structure results in a complex wave function index profile
which is to some extent less diffraction efficient that the zone
contain a single simple modulation profile. It is possible,
however, to compensate this inherently lower diffraction efficiency
by changing the exposure conditions of the two zones in order to
balance the levels of light diffracted by the two alternative
microstructures, such that they produce similar levels of
reflection in the finished hologram. Such compensation can be
achieved by precise control of the exposure times and beam ratios
of the various image components.
[0061] FIG. 11 demonstrates the mode of diffractive reflection from
the twin wavelength interference recording within the volume of the
hologram recording material. Such volume recording materials are
Silver halide fine grain gelatin based emulsion, photo-polymer
material, or dichromated gelatin, but are not necessarily limited
to those materials. Each fringe structure, which may in the case of
silver halide for example be recorded simultaneously with twin
laser wavelengths or alternatively consecutively with twin laser
wavelengths or by the use of a single wavelength with intermediate
chemical treatment between the consecutive exposures in order to
modify the thickness of the recording layer and thus adjust the
fringe frequency of the final dry hologram at reconstruction in
order to provide a plurality of colours. Such technique is known in
the art as pseadocolour holography.
[0062] FIG. 12 conversely shows the simple fringe structure
achieved with a single laser exposure and this effect could be
achieved by for example the use of a yellow krypton laser as
described above or by the use of a more distant laser line with
chemical adjustment of the reconstruction colour prior to laser
exposure or subsequent layer thickness adjustment at processing.
The spectrum of reconstructed image light in FIG. 12 shows a narrow
peak of wavelength distribution converse to the twin peaks shown in
FIG. 11. Referring again to FIG. 6 it is clear that both the red
and green cones will respond to the reflected yellow light in FIG.
12, with the effect that the viewer is unable to distinguish this
spectrum from that provided by the twin peaks represented in FIG.
11. Thus to the unaided human observer, it will be difficult to
detect any security information present in the holographic image
whereas electronic detection systems or visual examination with the
aid of filters or monochromatic light sources will readily detect
the covert information.
[0063] The effect of storing covert information in the hologram is
not limited of course to geometric patterns or text and could, for
example equally be used to disguise bar codes, fingerprint or other
biometric information, or any other two or three dimensional
graphics in a security hologram.
[0064] No doubt many other effective alternatives will occur to the
skilled person and it will be understood that the invention is not
limited to the described embodiments but encompasses modifications
apparent to those skilled in the art lying within the spirit and
scope of the claims appended hereto.
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